Polyethylene (PE) is consumed in the USA at the rate of over ten million metric tons every year. Presently, a variety of PE resins can be used to produce high stiffness pipe used in water, gas, and other fluid transport applications. Polyethylene pipe classified as PE-100, MRS 10, or ASTM D3350 typical cell classification 345566C is especially desirable for use under conditions requiring higher pressure ratings. To obtain a PE-100 classification, PE-100 pipe is required to meet certain standards specifying stiffness, resistance to slow crack growth, resistance to chemical attack, and low-temperature toughness (expressed as rapid crack propagation). Further, such pipe must meet a deformation standard that is determined under pressure at elevated temperatures.
It is also desirable for PE-100 pipe to exhibit toughness. This characteristic can be important in many polyethylene pipe applications, for example, where the pipe is buried underground or where the pipe is used to transport coarse or abrasive slurries. Accordingly, there is a need for a resin and a PE-100 pipe made therefrom that has improved physical properties and impact resistance properties.
With conventional processes and resins, there is a trade off between high stiffness and high environmental stress cracking resistance (ESCR). While either high stiffness or high ESCR items can be manufactured, conventional processes do not produce items having both high stiffness and high ESCR.
There is a need for a means of producing bimodal ethylene copolymers in which the component polymers have narrow molecular weight distributions and in which the comonomer can preferentially be placed in the high molecular weight portion, if desired. Further, there is a need for a cost-effective catalyst system for controlling the level of long chain branching (LCB) in PE to levels appropriate for resin applications such as HDPE film, pipe and bottles.